I think the two biggest reasons are her ice powers and definitely let it go for sure

This one!

Let it go, her magical ice powers and her "magical" fantasy white hair look. In a way Anna looks plain when compared to her.

And Let It Go.

I honestly think it's all of them.

And Let It Go

Who doesn't want to be an ice bender?

and her Ice Powers.

Aang- well it's funny because before Frozen came out, pretty much no one. Lol, really, NOBODY but a few people liked ice, they all liked fire and water but when Frozen was popular, suddenly everyone decided ice was their favorite despite a lot of these same people being the ones whining about how much they hate winter and snow. Ugh.

little kids always want what is portrayed/marketed as the 'best' character. that's why Elsa is more popular- she has a lot more to offer little kids who don't care about liking characters for semi-deep reasons beyond "omg powers her dress is so pretty I love her song!" than poor Anna, who is just the 'normal' one. It's like how Harry is more popular than Ron, Daenerys and Arya are more popular than Sansa and Cat, La Muerte is more popular than Maria, etc.

^^
Little kids don't just like what's marketed more..that's crazy talk...that's not how marketing works at all
U market everything, Disney markets every film and character, then depending on the response it dies down, or they continue the marketing, it's about supply and demand, u can't make a child like something, they either like it or they don't and Disneys job is to put out what sells and continue to put out until it stops selling, because making merchandise and marketing something costs money, and it only pays off if people buy it!

Regarding this question....I actually read an article done by a child psychologist about why children are responding so well to elsa and alot of it is relatebility especially in children, because children generally feel like that have to hold in alot, like they can't just play and act crazy, they have to sit down, be quiet, control themselves in many situations...alot of childhood is spent learning about self control...so elsa inner struggle is apparently extremely relatable to young kids, especially kids with ADHD and other similar issues

And Let it Go